What is Compression Dictionary Transport?

Compression Dictionary Transport (CDT) is an IETF standard, RFC 9842 from September 2025. It lets Brotli and Zstandard compress a response against a dictionary the browser already has, instead of compressing from scratch. The dictionary encodings are dcb (Dictionary-Compressed Brotli) and dcz (Dictionary-Compressed Zstandard). A common case is a new app version compressed against the previous version, so repeat visitors download only the difference.

How does this checker test my site?

Enter a URL and the tool fetches it from the Cubbbix server, then reads the response headers and the page HTML. It looks for a dictionary link, checks whether the dictionary itself serves a Use-As-Dictionary header, and reads the encoding, Vary and CSP headers. The fetch runs server-side on purpose: a browser fetch from this page would be blocked by CORS on most sites.

Why is my score low if my site already uses Brotli?

Brotli alone is not CDT. Brotli compresses each response from scratch, while CDT compresses against a shared dictionary. To score on the dictionary checks you need a dictionary link on the page or in a Link header, a dictionary response that carries Use-As-Dictionary with a match pattern, and a Vary header that includes accept-encoding and available-dictionary.

What exactly must the Vary header contain?

For any cacheable dictionary-compressed response, Vary must list both accept-encoding and available-dictionary. Without that, a cache can serve a dictionary-compressed response to a browser that never asked for it, or serve a response compressed with the wrong dictionary.

Does CDT work in every browser?

No. Today it works in Chromium browsers (Chrome and Edge). Firefox has an implementation in progress and Safari has not announced support. That is safe rather than risky: browsers that do not advertise dcb or dcz simply receive normal Brotli, so CDT is a fast path for supporting browsers, not a dependency.

Why is Available-Dictionary not scored here?

Available-Dictionary is a request header the browser sends when it already holds a matching dictionary. A server-side check cannot observe what a visitor browser has stored. That client-side half belongs to the CDT Browser Support Test, which is the next tool in this family. This checker scores everything a server can prove from headers.

Is my URL or my site data stored?

No. The URL is fetched once for the check and nothing about the result is stored on Cubbbix. Your site content is never shown, only the headers that decide the score.

Compression Dictionary Readiness Checker

Test any website for Compression Dictionary Transport: dictionary links, Use-As-Dictionary, Vary headers, Brotli, Zstandard, DCB and DCZ. Free, no sign-up.

Website URL

Fetched server-side from Cubbbix, so no CORS limits. Only public http and https URLs are accepted; private addresses are refused. Nothing is stored.

0/100

CDT readiness

Check	Weight	Result	Detail

Reusable assets found

Response headers seen:

How the score is calculated

The score is public on purpose: every check and its weight is listed here, so a low score tells you exactly what to fix. Encoding checks read the page response itself.

Check	Points	What passes
Secure context (HTTPS)	10	Page served over HTTPS. Dictionaries only work in secure contexts.
Brotli encoding	10	Page Content-Encoding includes br.
Zstandard encoding	10	Page Content-Encoding includes zstd.
Dictionary link present	20	A Link header or link tag with rel="compression-dictionary".
Dictionary serves Use-As-Dictionary	20	The dictionary response carries Use-As-Dictionary with a match pattern.
Correct Vary header	10	Vary includes both accept-encoding and available-dictionary.
CSP allows dictionary fetch	10	No CSP, or connect-src allows the dictionary host.
DCB or DCZ actually served	10	Page Content-Encoding includes dcb or dcz.

Grades: 80 to 100 is ready, 40 to 79 is partial, below 40 is not ready. Available-Dictionary negotiation is deliberately not scored: it is a request header the browser sends, and only a client-side test can observe it. That test is the next tool in this family.

What Compression Dictionary Transport actually does

Normal compression starts from nothing on every response. CDT lets the browser reuse something it already downloaded as the dictionary for the next response. Ship app-v1.js, then serve app-v2.js compressed against v1 as its dictionary, and a repeat visitor downloads only the difference. The same idea works for shared page templates: one dictionary built from your common HTML, reused across every page. In one lab test reported by Cloudflare, a 272 KB script compressed to 92.2 KB with gzip and to 2.6 KB with dcz against its previous version. Your numbers depend on how much actually changed, but for frequent small deploys the ratio is the whole point.

Dictionaries must be same-origin with the resources they compress, dictionary responses must be served over HTTPS, and a dictionary-compressed response must carry the right Vary header or caches will serve it to the wrong browsers. Get those three rules right and the rest is plumbing: publish a dictionary, point at it with a link, and serve dcb or dcz to browsers that ask for it.

How to fix a failing check

Serve the site over HTTPS if it is not already. Nothing else in CDT works without it.
Publish a dictionary file and reference it with a Link header or a link tag: rel="compression-dictionary".
Serve that dictionary with Use-As-Dictionary and a match pattern covering the resources it applies to, for example match="/js/*".
Add Vary: accept-encoding, available-dictionary to every dictionary-compressed response.
If you set a Content Security Policy, make sure connect-src allows the dictionary URL, or the browser will refuse to fetch it.
Precompress one delta per release for your largest versioned bundle, then measure transfer size in DevTools on a repeat visit.

Frequently Asked Questions

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